Condenser microphones



March 21, 1967 w. D. CRAGG ETAL 3,310,628

CONDENSER MI CROPHONES Filed July 26, 1963 3 Sheets-Sheet 1 HT/ HT? l/T Inventors W/LL /AM 0. CRA G6 March 21, 1967 w. D. CRAGG ETAL 3,310,623

7 CONDENSER MICROPHONES Filed July 26, 1965 3 Sheets-Sheet 2 LOG is 5b 160 500 cmzs m sfm/vo InvenlorS WILL/AM 0. C'IQAGG ERNEST R. COCKBA/N 5 Sheets-Sheet 5 6/ rim 7W sA mm m u MCO? A w a C R 0 r 4 5 m m March 21, 1967 w. D. CRAGG ETAL CONDENSER MICROPHONES Filed July 26, 1963 response overall.

Uited States Patent The present invention relates to electrostatic microphone arrangements, and more particularly to arrangements of two or more microphone capsules of particular directional properties in which the electrical outputs of the separate capsules are combined in such a manner that the microphone arrangement has a desired directional It has been found that small electrostatic microphones or microphone arrangements having a cardioid shaped polar response are highly sensitive to slight air movements and mechanical handling in response to which they produce large low frequency signals which can be of sufficient magnitude to distort or mask the wanted acoustical signals. This sensitivity to unwanted signals is inherent in the pressure gradient mode of operation required to obtain the cardioid directionality; a pressure mode of operation would result in the sensitivity to this form of excitation being reduced by up to 40 decibels relative to a given strength of the wanted signal. It has therefore been found desirable to provide an electrostatic microphone arrangement which has a cardioid directionality for incident pressure variations corresponding tofrequencies greater than 100 cycles per second and a smooth transition from cardioid directionality to an omni-directional response for slower rates of pressure variations until for pressure variations corresponding to frequencies of less than 40 cycles per second, the arrangement is substantially free from any directional effects and virtually in a pure pressure mode of operation.

According to one aspect of theinvention, there is provided an electrostatic microphone arrangement comprising a first transducing element responsive to pressure gradient and a second transducing element responsive to pressure, wherein the electrical outputs from the transducing elements are combined in such a manner that the -microphone arrangement has a directionality which varies with the rate of incident pressure variations.

According to another aspect of the invention, there is provided an electrostatic microphone arrangement comprising a transducing element responsive to pressure gradient having a decreasing sensitivity below a turnover frequency and a transducing element responsive to pressure having a decreasing response above said turnover frequency, wherein tthe electrical outputs from the transducing elements are combined in such a manner that the arrangement has a directionality which varies with the frequency of incident sound pressures.

According to yet another aspect of the invention, there is provided an electrostatic microphone arrangement comprising a first transducing element responsive to pressure gradient mounted in the open end of a tube and a second transducing element responsive to pressure mounted in .the other and closed end of said tube, a space inside said tubebetween the rear face of said first transducing element and the diaphragm of said second transducing element, and holes in the portion of the tube surrounding said space, wherein the electrical outputs from the transducing elements are combined in such a manner that the microphone arrangement has a directionality which varies with the rate of incident pressure variations.

Embodiments of the invention will now be described with reference to the accompanying drawings in which:

FIG. 1 shows a diagram of a basic circuit in which the electrical outputs of two different microphone capsules are combined;

FIG. 2 shows a diagram of a circuit in which there is provision for the application of different polarizing potentials to the two capsules whose outputs are combined;

FIG. 3 shows a diagram of a transformer bridge circuit;

FIG. 4 illustrates the frequency response characteristic of the sensitivity of one possible arrangement;

FIG. 5 illustrates one form of construction of the electrostatic microphone arrangement; and

FIG. 6 illustrates another form of construction of the electrostatic microphone arrangement.

.at the rate of 6 decibels per octave below, say a turnover frequency of cycles per second. The pressure capsule C is connected in parallel with the pressure gradient capsule C the output from which is arranged to fall off at the rate of 6 decibels'per octave above, say, a turnover frequency of 25 cycles per second. By suitable control of the sensitivities of the two capsules, the frequency responses, as illustrated in FIG. 4, by the graph of the logarithm of the combined output voltage V as a function of frequency can be arranged to result in a combined response which is substantially level down to less than 25 cycles per second for sound pressures incident on the two capsules from a direct normal to the diaphragm of the pressure gradient capsule. In this context, it is to be understood that whenever in this specification, the term frequency" is used, it is merely a matter of convenience for purposes of explanation and definition. The use of the term frequency is intended to be representative of not merely the frequency of sound pressures varying in simple harmonic motion but also the equivalent rates of change of any pressure incident on the microphone arrangement, including pressure variations of a non-periodic nature such as explosion blast waves. As is well known, a non-periodic function can be analysed mathematically into component sinusoidal functions to any desired degree of accuracy for a selected time portion of the non-periodic function, but the analysis of the response of a system of dissipative components and energy storing components to such a non-periodic function is generally rather more complex than the equivalent aproach to its analysis in terms of a periodic function of a particular recurrence frequency.

An alternative basic circuit is illustrated by the diagram of FIG. 2. In this circuit, provision is made for applying different polarizing voltages HT1 and HT2 to the pressure capsule C and the pressure gradient capsule C respectively. As for the circuit illustrated by the diagram of FIG. 1, the electrical outputs of the two capsules are still elfectively in parallel and applied to the grid of the valve V, but the unidirectional polarizing voltages applied to them are separated by the blocking capacitor C. Different polarizing voltages are provided to enable either of the microphone capsules to be adjusted to have a sensitivity in the level range of its frequency response such that the frequency response of the combined electrical output is substantially level for one direction of incidence of the pressure variations at least for that range of frequencies on either side of the two turnover frequencies in of a capsule is reduced by a factor of /2, then it would be undesirable in a high quality sound system for the turnover frequencies to be spaced by more than one octave; the rates of variation of incident pressure corresponding to these turnover frequencies would therefore not differ by a factor greater than two.

The desired falling off in response at the upper end of the frequency range of the pressure capsule C may be obtained by suitable choice of the resistor R, shown in FIGS. 1 and 2, which form part of an integrating circuit together with the capacitance of the capsule C Similarly, a differentiating circuit may be used to obtain the desired falling off in response at the lower end of the frequency range of the pressure gradient capsule C An example of such a differentiating circuit is illustrated by the resistor R andthe capacitor C shown in FIG. 2. Although this circuit arrangement causes differentiation of the electrical outputs of both of the microphone capsules, it should be noted that this differentiation is effective for the pressure capsule C at a somewhat lower frequency than for the pressure gradient capsule C because of the presence of the resistor R which is effectively additive to the resistor R and thereby increases the time constant of the differentiating circuit operated into by the pressure capsule C relative to the time constant of the differentiating circuit operated into by the pressure gradient capsule C It will be readily apparent that for this purpose the values of the resistance R and R1 are preferably of the same order of magnitude.

A different approach to the problem of providing the desired frequency response characteristics enables advantage to be taken of the requisite reductions in bandwidth of the separate capsules to obtain an increase in the sensitivity of the combination. The falling off in the low frequency range of the pressure gradient capsule is procured by increasing the stiffness of its diaphragm. Since this capsule requires to be resistance controlled for the frequency range for which it is intended to have a level response characteristic, adjustment of the diaphragm stiffness causes the turnover frequency to be varied, an increase in stiffness being accompanied by an increase in turnover frequency. An increased stiffness prevents the diaphragm from collapsing onto the fixed electrode of the microphone capsule due to the increased attraction resulting from the application of a greater polarizing voltage, which also has the effect of increasing the sensitivity for frequencies greater than the turnover frequency. Another possible method of obtaining an increased sensitivity consists of increasing the ratio of diaphragm stiffness to control resistance by decreasing the control resistance with the effect of a corresponding increase in the sensitivity for frequencies above the turnover frequency.

The pressure capsule requires to be stiffness controlled for the range of frequencies for which a substantially level response is desired; in a normal pressure microphone the stiffness of the diaphragm and the cavity formed between the diaphragm and the fixed electrode is arranged to be dominant over other acoustical impedance elements to the upper limit of the desired frequency range, which may be in excess of 10,000 cycles per second. Above the frequency at which the acoustic reactance is substantially equal in magnitude to the acoustic resistance of the material behind the diaphragm, the resistance controls the frequency response characteristic and causes the electrical output to fall off at the rate of 6 decibels per octave. Adjustment of either of these two acoustical impedance elements enables the turnover frequency to be reduced to the relatively low value which is required. The acoustical resistance, which for a normal pressure capsule of about in diameter is of the order of 250 ohms has to be increased a hundredfold, i.e. to about 25,000 ohms, if the diaphragm stiffness is to be kept constant. An increase in acoustical resistance of this order can be attained by using an acoustically transmissive fixed electrode between the diaphragm and a layer of material of high specific acoustical resistance, such as porous polyvinylchloride, of suitable thickness. Alternatively or complementarily, the reduction in turnover frequency may be effected by a reduction in the stiffness of the diaphragm by reducing its tension and thereby gaining an increase in sensitivity for a given polarizing voltage, which will set the lower limit to the tension needed to prevent the diaphragm collapsing onto the fixed electrode under the force of electrostatic attraction between the :fixed electrode and the diaphragm. A further factor affecting the frequency response characteristic of the pressure capsule is the possible desideratum that the response of the com plete arrangement shall fall off below a third turnover frequency of lower value than either of the other two turnover frequencies; this result may be obtained by providing a suitable small acoustic leak between the front and rear sides of the diaphragm.

The methods outlined above for controlling the turnover frequencies of the separate capsules provide fallingoff rates in response of either 6 or 12 decibels per octave according to whether only acoustical, only electrical means or both acoustical and electrical means are used for controlling the turnover frequencies. The choice is a matter for individual preference and may be influenced by the desirability to achieve the aimed-at change of. directionality over the frequency range of the arrangement without recourse to non-standard types of microphone capsules, the desirability to exploit the reduction in bandwidth of the separate capsules in order to increase the sensitivity of the microphone arrangement as a whole or the desirability of achieving a change-over rate of 12 decibels per octave between the two capsules in the frequency ranges in which one of the two capsules is intended to be the dominantly responsive one and substantially determinative of the directionality of the combination.

For those cases in which it is held appropriate to use only acoustical modifications or adjustments of the separate capsules to obtain the requisite frequency response characteristics for each of them, a high frequency bridge circuit may be used to supply polarizing voltages to the two capsules and combine their electrical outputs. An example of such a circuit is illustrated by the diagram shown in FIG. 3. A transistor oscillator T energizes the primary winding of a transformer M at a frequency rather greater than the highest frequency of pressure variations to which the microphone arrangement is to be sensitive. For the sake of simplicity and clarity, details of the oscillator circuit relating to bias supplies for the transistor have been omitted from the diagram since these may be arranged in a number of conventional ways. Omitted for s1milar reasons are the sources of unidirectional potential which may optionally be provided to augment the high frequency supply for applying polarizing potential to the nncrophone capsules. These D.C. potentials may be introduced into the microphone circuit in many different known ways. The secondary winding of the trans-former 1s center-tapped and provides two ratio arms of the high frequency bridge, the other two arms of which are provided by the two microphone capsules C and C respectively. In the absence of pressure variations deflecting either of the diaphragms of the capsules from their equil1br1um positions, the bridge is balanced and thus no unbalance high frequency voltage appears across the inductor L which is connectedbet-ween the center point of the secondary winding of the transformer M and the junction of the capacitances representing the two microphone capsules; in practice, this junction would be the preferred point to be grounded in the circuit. To obtain the balance of the bridge circuit in the static condition, it will usually be necessary to connect a variable trimming capacitor in parallel with either or both of the capsule-s to compensate for any difference in capacitance which there may be between them either by design or due to normal manufacturing tolerances. For greatest sensitivity of the bridge, it is desirable that the values of'the capacitance of any such capacitors be kept as low as is consistent with their being adequate to compensate for the largest possible ditference in capacitance between the two capsules. If the bridge circuit is initially balanced, then any pressure variation within the frequency range of either capsule will cause a change in the corresponding capacitance, which in'turn will cause an out-of-balance high frequency voltage to appear across the inductor L. This high frequency voltage is amplitude modulated by the capacitance changes, but comprises only the side bands of the modulation products. By feeding this unbalance voltage symmetrically through equal portionsof the secondary winding of the transformer M, the carrier high frequency voltage is re-inserted into the unbalance voltage and the sum of these voltages is applied to the two rectifying diodes D1 and D2 which have a common load consisting of the two equal resistances R1 and R2 shunted by capacitors C1 and C2. The demodulated voltage then appearing between the junction of the two resistances and the junction of the capacitances is within practical limits proportional to the capacitive unbalance of the capsules C and C The direct current return path for the two diodes is provided by the inductor L. In the balanced condition of the bridge, the amplitudes of the carrier frequency vo-ltages applied to the two rectifying diodes are equal to each other. Since the oppositely connected diodes are working into equal load resistances, the unidirectional poten tials developed across these load resistances are equal to each other and there is no unidirectional potential difference between the junction of the resistors and the junction of the capacitors; the output voltage between these two points in the circuit consists therefore only of the alternating component due to the aforementioned capacitive unbalance. It should be noted, that when the two microphones are used as shown in the circuit illustrated by the diagram of FIG. 3, their diaphragm displacements must be arranged in a manner that the two separate capacitance variations in response to any pressure variation are in mutual phase opposition. One method of achieving this phase reversal consists of interchanging the positions of the fixed electrode and the diaphragm of the pressure capsule already described, i.e. by arranging the diaphragm behind the acoustically transmissive fixed electrode and in front of the cavity containing the layer of material of high specific resistance. Since there is a high acoustic resistance behind the diaphragm, a certain amount of acoustical resistance in the fixed electrode is tolerable and it can therefore be made of perforated sheet metal having holes of 0.001 in diameter or consist of a sintered metal plate having an acoustical resistance in the range of 100 to 10,000 ohms; in either form, the fixed electrode can therefore act as an effective dust excluder. If a reduction in the low frequency response is desired, then, as already described, this may be obtained by providing a small acoustic leak between the front and rear of the diaphragm.

FIGS. and 6 show two practical forms of the arrangement of the two microphone capsules in a microphone as sembly. The two forms differ only in the method of housing the two capsules in one assembly and in being illustrated in a view of a diametrical section in FIG. 5 and the right hand half of FIG. 6 and an external view for the left hand half of FIG. 6. The two electro-acoustical transdu'cing elements of each assembly consist of a pressure gradient capsule G and a pressure capsule P, each having a diaphragm 1 and a fixed electrode 2, behind which there is the acoustical impedance element 3 which comprises component elements of acoustical resistance and acoustical stiffness (or capacitance) appropriate to the function of the respective capsule. The rear face 4 of the pressure gradient capsule is open to the external pressure variation, while the rear face of the pressure capsule is blocked by a plate 5 either completely or at least to the extent of providing no more than a small leak between the front and rear sides of the diaphagm of the pressure capsule for reducing its sensitivity to pressure variations of very low frequency, as already referred to. Furthermore, if either form of the arrangement is intended to be used in a high frequency bridge circuit such as illustrated by the diagram of FIG. 3, the relative positions of the diaphragm 1 and the fixed electrode 2 of the pressure capsule P will need to be interchanged. Since the pressure capsule makes a significant contribution to the combined electrical output only at low frequencies, the wavelengths of the pressure variations to which it is effectively sensitive will be relatively long in comparison with the physical dimensions of the assembly and hence the position of the pressure capsule relative to the pressure gradient capsule is not critical.

One suitable structure is shown in FIG. 5, in which the pressure gradient capsule is mounted in the open end of a tube 6 and the pressure capsule is mounted in the other end of the tube which may be closed by a portion 7 of the stand or handle on which the assembly is mounted. The two capsules are mounted to leave a space between the rear face 4 of the pressure gradient capsule G and the diaphragm 1 of the pressure capsule P. The portion of the tube 6 surrounding this space is provided with a number of holes 8 to permit external pressure variations to be freely communicated to the rear face 4 of the pressure gradient capsule G and the diaphragm 1 of the pressure capsule P.

FIG. 6 illustrates an alternative form of construction in which the tube 6 of FIG. 5 is replaced by a ring 9 which fits around the pressure capsule P and is mounted on the support 7. This ring 9 has a member of paraxial projections 10 to which the pressure gradient capsule G is atlixed. The apertures 8 between these projections correspond to the holes 8 of FIG. 5 and serve to permit external pressure variations to be freely communicated to the space between the rear face 4 of the pressure gradient capsule G and the diaphragm 1 of the pressure capsule P.

It is to be understood that the foregoing description of specific examples of this invention is not to be considered as a limitation on its scope.

What we claim is:

1. An electro-static microphone arrangement comprismg:

a first transducing element principally responsive to pressure-gradient to produce a first signal;

a second transducing element principally responsive to pressure to produce a second signal; and

means to combine said first and second signals to produce an output signal including:

an oscillator providing a carrier signal;

a bridge, said first and second transducing elements each forming one arm of said bridge;

a transformer, including a primary and a secondary winding, said secondary including a center tap dividing said secondary into two equal parts, said parts serving as the outer two arms of said bridge, said tranformer coupling said carrier signal to said bridge, an inductor connected between said center tap of said secondary winding and the junction of said transducing elements, a voltage appearing across said induct-or whenever the pressure acting upon said transducing elements varies;

two rectifying diodes; and

means coupling said voltage across said inductor to said diodes including a portion of both arms of said secondary winding.

2. An electro-static microphone arrangement according to claim 1 wherein said first transducing element in cludes an acoustic resistance and a diaphragm, the stiffness of said diaphragm providing an acoustic reactance which is approximately equal in magnitude'to said acoustic resistance at said first frequency, and said second transducing element includes an acoustic resistance and a diation to be communicated to said transducing elephragm, the stiffness of said diaphragm and at least part merits. of said second transdu-cing element providing an acoustic reactance which is approximately equal in magnitude to References filled by the Examine said acoustic resistance of said second transducirzg ele- 5 UNITED STATES PATENTS meat at said Swmd frequency 2 1.84 247 12/1939 Baumzwei er 179-1 3. An electr c-static microphone arrangement ac-cord- 22]9676 10/191) Barber 332 3 ing to claim 2 further including: 2678967 5/195'4 Gmsskogf 179 1 a supporting member, said first transducing element be- 5 4/1957 Grosskopf j ing mounted at one end of said member and said 10 2920140 1/1960 Mo'roan second transducing element being mounted at the D n other end of said member; FOREIGN PATENTS an empty space within said supporting member between 881, 12/1958 Gffiat Britainsaid transducing elements; and 1 T T i apertures in the part of said supporting member sur- 5 KAJWUJQLN CLAFFY Pnmal'y Exammw' rounding said space to permit external pressure varia- A. H. GESS, Assistant Examiner. 

1. AN ELECTRO-STATIC MICROPHONE ARRANGEMENT COMPRISING: A FIRST TRANSDUCING ELEMENT PRINCIPALLY RESPONSIVE TO PRESSURE-GRADIENT TO PRODUCE A FIRST SIGNAL; A SECOND TRANSDUCING ELEMENT PRINCIPALLY RESPONSIVE TO PRESSURE TO PRODUCE A SECOND SIGNAL; AND MEANS TO COMBINE SAID FIRST AND SECOND SIGNALS TO PRODUCE AN OUTPUT SIGNAL INCLUDING: AN OSCILLATOR PROVIDING A CARRIER SIGNAL; A BRIDGE, SAID FIRST AND SECOND TRANSDUCING ELEMENTS EACH FORMING ONE ARM OF SAID BRIDGE; A TRANSFORMER, INCLUDING A PRIMARY AND A SECONDARY WINDING, SAID SECONDARY INCLUDING A CENTER TAP DIVIDING SAID SECONDARY INTO TWO EQUAL PARTS, SAID PARTS SERVING AS THE OUTER TWO ARMS OF SAID BRIDGE, SAID TRANFORMER COUPLING SAID CARRIER SIGNAL TO SAID BRIDGE, AN INDUCTOR CONNECTED BETWEEN SAID CENTER TAP OF SAID SECONDARY WINDING AND THE JUNCTION OF SAID TRANSDUCING ELEMENTS, A VOLTAGE APPEARING ACROSS SAID INDUCTOR WHENEVER THE PRESSURE ACTING UPON SAID TRANSDUCING ELEMENTS VARIES; TWO RECTIFYING DIODES; AND MEANS COUPLING SAID VOLTAGE ACROSS SAID INDUCTOR TO SAID DIODES INCLUDING A PORTION OF BOTH ARMS OF SAID SECONDARY WINDING. 